Epoxy resins are high performance materials used in a wide variety of applications including protective coatings, adhesives, electronic laminates (such as those used in the fabrication of computer circuit boards), flooring and paving applications, glass fiber-reinforced pipes, and automotive parts (including leaf springs, pumps, and electrical components). In their cured form, epoxy resins offer desirable properties including good adhesion to other materials, excellent resistance to corrosion and chemicals, high tensile strength, and good electrical resistance. Two challenges associated with the use of epoxy resins are the brittleness of the cured epoxy resins and the need to heat many curable epoxy compositions enough to prepare and blend and shape them but not so much as to cure them prematurely.
Poly(arylene ether) resin is a type of plastic known for its excellent water resistance, dimensional stability, and inherent flame retardancy. It is known that the brittleness of epoxy resins can be reduced the addition of poly(arylene ether)s. For example, U.S. Pat. No. 4,912,172 to Hallgren et al. describes a composition including a polyphenylene ether having a number average molecular weight of at least about 12,000 and a specific epoxy material. However, relatively high temperatures are required to dissolve the polyphenylene ether in the epoxy resin.
As another example, U.S. Pat. No. 5,834,565 to Tracy et al. describes compositions including an epoxy resin and a poly(arylene ether) having a number average molecular weight less than 3,000. The low molecular weight poly(arylene ether) is easier to dissolve in the epoxy resin than higher molecular weight poly(arylene ether)s. However, the products obtained on curing these compositions are not as tough as those prepared with higher molecular weight polyphenylene ethers.
Known curable compositions comprising poly(arylene ether)s and epoxy resins thus appear to present a trade-off between toughness of the cured product and ease of preparing and processing the curable composition. When a high molecular weight poly(arylene ether) is employed, the cured product is very tough, but elevated temperatures are required to dissolve the poly(arylene ether) in the epoxy resin. On the other hand, when a low molecular weight poly(arylene ether) is employed, it is possible to dissolve the poly(arylene ether) in the epoxy resin at a lower temperature, but smaller improvements in toughness are observed in the cured product.
There remains a need for curable epoxy compositions that can be processed at low temperature yet be extremely tough (less brittle) after curing. This need is particularly great in the field of circuit board manufacturing, where improvements in toughness are needed, but existing processes and equipment cannot accommodate curable compositions that must be prepared and maintained at high temperatures to maintain dissolution of a poly(arylene ether).